After finished installing you new windows OS, it will leave a folder named as Windows.Old. It is also used a big size of your hard drive. You cannot easily delete it. So, follow this instructions.
STEP TO DELETE WINDOWS.OLD
1. Open Disk Cleanup program. Go to Start Menu >> All Programs >> Accessories >> System Tools >> Disk Cleanup
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2. Then, click Clean up system files >> Ok
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3. After that, check Previous Windows Installation(s), then click Ok.
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4. Just wait til' finish.
Now, folder Windows.Old is deleted. :)